The goals of the operative procedures known as total knee replacement, partial knee arthroplastic or the uni-condyler procedure (TKR/TKA/UNI) are to restore pain free walking capability in patients having worn or diseased knee joints. In these common orthopedic procedures, damaged surfaces of the knee bones are replaced with prosthetic joint implants, installed at the distal femoral and proximal tibial bone ends, after appropriate cuts of the tibia and femur bones have been made by the surgeon. In order to ensure correct functionality of the knee and a long working life of the implants, all of the implanted components must be positioned with high precision in relation to the mechanical axis of the leg, the anatomical axes of the bones and the mating surfaces between the bones and the prostheses.
A number of geometric anatomical relationships must be fulfilled for the reconstructed knee to function correctly and with longevity. These relationships can be observed by reference to FIG. 1, which is a schematic illustration of the optimal anatomic position of the bones of the leg, as viewed from the anterior/posterior (A/P) direction. The relationships are as follows:                (a) The mechanical axis of the leg 10, running from the center of the acetabulum head 20 of the femur to the center of the ankle 22, should pass through the defined middle of the knee joint 12.        (b) The femur anatomical axis 14 should be inclined at an angle of about 7° with the leg mechanical axis. This angle is known as the valgus angle 16.        (c) The tibia bone axis 18 should be collinear with the mechanical axis 10 of the leg.        (d) The mechanical axis of the leg, the axis of the femur bone and the axis of the tibia bone should lie in one plane, when the leg is completely straightened. This plane is that of the plane of the drawing of FIG. 1.        
When accurately performed, the procedure should arrange the entire knee joint so that forces are transferred through the component parts of the leg along a well-defined mechanical axis of the leg, from the center of the acetabulum head 20, through the middle of the knee joint 12, and to the ankle 22.
In order to satisfy all these requirements, the surgeon must be able to fulfil a number of conditions with respect to the correct orientation of the planes cut at the ends of the bones for the installation of the implants:                (i) Control of the orientation of the cutting planes relatively to the mechanical axis of the leg.        (ii) Control of the orientation of the tibial cutting plane.        (iii) Control of the orientation of the femoral cutting plane.        
With prior art techniques as currently used, measurements of the leg mechanical axis are not performed intraoperatively. Generally the surgeon performs a visual estimation of the required cuts, relying on his senses and experience, or is assisted in this task by the use of mechanical rods laid along the estimated axes of the bones. This task is problematic because of the difficulty of accurately estimating the orientation of the cutting planes, each having two rotational degrees of freedom, relative to the leg's mechanical axis, the tibia axis and the femur axis, and one translational degree of freedom relative to the bone positions, in a situation when almost all of the leg structure is covered by soft tissue and no axis measurements are performed. In a typical procedure, a complete X-ray image of the leg is used to enable the surgeon to estimate the 7° valgus angle, which is the angle with the perpendicular to the femur axis at which the cut of the end of the femur is made.
The instrumentation currently available can generally give an indication only of large errors in such a cutting plane orientation. Thus, although the simple implantation guidance instrumentation currently used, such as direction gauges and mechanical guide rods attached to the leg, is of some assistance to the surgeon in achieving the correct alignment of the leg axis and the implants, the success thereof depends very largely on the surgeon's experience. According to studies such as that reported in “Our Experiences with Robot Assisted Surgery in Comparison with Navigation and Manual Technique in Total Knee Arthroplasty”, by S. Mai, et al., Computer Assisted Orthopedic Surgery, 2002, and that reported in “Acrobot system for total knee replacement” by M. Jakopec, et al., published in Industrial Robot, Vol. 30, No. 11, pp. 61-66, 2003, over one third of such operations were found to have deviations from ideal prosthesis alignment which resulted in premature failure of the repaired joint. It is known that there is a high correlation between survival rate of the implant, and misalignment thereof. According to other studies, it is believed that the failure rate of such knee joint operations, as defined by the need for a corrective operation within 2 years, is up to 15% of the total number performed, half of which are due to misalignment problems. Because of the complexity of the situation produced, a corrective operation is often several times more costly than a regular operation. The main reasons for such high misalignment rates is that the current instrumentation usage depends highly on the surgeon's experience.
Many different approaches have been proposed in order to reduce the failure rate in this field of orthopedics. Navigation systems have been proposed, both with or without CT correlation. These systems are variously based on CT scans, fiducial marks, the matching of bone surfaces with template bones, and x-ray imaging, together with state-of-the-art cutting tools and the use of instruments such as robots. One such prior art system uses tracker markers on the upper leg, which are followed by the navigational system as the leg is swung, the center of motion of these markers defining the center of the hip joint. The same procedure performed with tracking markers on the lower leg enables the center of the knee joint to be determined by the navigation system, since the tibia revolves in an arc with the center of the knee joint at its center. The ankle is then designated with a touch pointer to enable the tibia axis to be defined. Finally, the cutting jigs mounted on the bones are also provided with tracking markers to define their position to the navigator system. This system thus is able to define the axes of the bones, their joint centers and the position at which the cut is to be made using the cut guidance jig. The main drawback of such systems is their complexity, the longer procedure time required, their price and their large size, all or some of which may be contributory factors as to why there has so far been small usage of such systems in these procedures, when compared to the use of conventional TKA tools.
There therefore exists a need for a new system, method and associated accessories for enabling the performance of TKR/TKA/UNI procedures to be executed with higher precision and success rate than those currently used, and with less dependence on the professional subjective skills and judgement of the surgeon performing the procedure.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.